The invention relates in general to sensors used to detect properties of a fuel and, more particularly, to a sensor for detecting the complex impedance of a fuel.
The property of a gasoline, such as its conductivity or dielectric constant, are often important for operation of a motor vehicle. Such constants can be used to provide the concentration of ethanol in a gasoline and can also determine the amount of water mixed in with the fuel. For example, experimental data shows that the fuel dielectric constant is directly proportional to the ethanol concentration but relatively insensitive to water contamination, while fuel conductivity is very sensitive to water concentration. Thus, for these applications and others, there is a need for a fuel sensor that precisely measures the impedance of fuel.
Current sensor designs have problems handling small capacitance measurements, requiring a relatively large sensing element to increase the signal-to-noise ratio. Further, instead of separately measuring resistance and capacitance, the designs measure total impedance, requiring a relatively high frequency in the 10-100 MHz range to reduce the conductivity impact. Two excitation frequencies are then needed to complete the measurement, low frequency for resistance measurements and high frequency for capacitance measurements.
The present invention is a sensor design with a small sensing element to minimize sensor package size, which is capable of measuring both resistance and capacitance using a single, low excitation frequency. The low excitation frequency reduces circuit radiation and, hopefully, the cost of components. The sensor is capable of handling very small capacitance and resistance values with high accuracy.
Specifically, the apparatus for determining the complex impedance of a fuel includes a sensing element in contact with the fuel, means for exciting the sensing element with an excitation signal of a predetermined frequency to generate an induced signal, means for generating a phase signal using the induced signal, and means for producing a magnitude signal using the induced signal. The phase signal is indicative of the phase of the complex impedance, while the magnitude signal is indicative of the magnitude of the complex impedance. The apparatus can include means, such as a microcontroller, for calculating either the resistance or the capacitance or both using the magnitude signal and the phase signal. The engine controller can receive the magnitude signal and the phase signal and calculate these quantities.
A typical sensing element comprises two spaced electrodes. Preferably, one or more direct current (DC) block capacitors remove DC components from the excitation signal and the induced signal.
The excitation signal is generally a sinusoidal voltage supplied by a sinusoidal source. Preferably, the predetermined frequency of the excitation signal is in a range of 10 kHz to 100 kHz. The signal induced by the sensing element is an induced current signal, which is preferably converted to an induced voltage signal by a current-to-voltage converter. In a preferred embodiment, the current-to-voltage converter includes an operational amplifier with an adjustable gain. The adjustable gain is then used to change the resolution of the induced voltage signal based upon the resolution of the magnitude signal. If the signal resolution is not high enough, the gain can be adjusted. The adjustable gain can include a plurality of selectable impedances. Then, one embodiment of the invention can include means for selecting at least one of the plurality of selectable impedances to change a resolution of the induced voltage signal based upon a resolution of the magnitude signal. This selection can be performed by, for example, a microcontroller or the engine controller.
The means for generating the phase signal can include means for comparing the induced signal to the excitation signal the phase signal. In one embodiment of the invention, this comparison the comparison means includes a first comparator for generating a first square wave corresponding to the induced signal, a second comparator for generating a second square wave corresponding to the excitation signal and a pulse width modulator for comparing the first square wave and the second square wave and generating the phase signal. The phase signal thus has a duty cycle representing the phase of the complex impedance.
The means for producing the magnitude signal preferably includes some type of peak detector. The illustrated embodiment describes a full-wave rectifier for receiving the induced signal and producing a rectified signal and a low pass filter coupled to the full-wave rectifier for receiving the rectified signal and producing the magnitude signal. A differential amplifier can be coupled to the low pass filter for amplifying the magnitude signal.
One desirable embodiment includes a switch that selects a reference signal to use in the apparatus such that means for producing the magnitude signal produces a reference magnitude signal. This reference magnitude signal can be used to adjust the magnitude output for temperature variations in the sensor apparatus. More specifically, the apparatus can include means for calculating an adjustment factor, such as the microcontroller or engine controller previously mentioned. The adjustment factor represents a change in a magnitude of the reference magnitude signal from a reference voltage. This change in magnitude varies with changes in the temperature. Thus, the means for calculating the adjustment factor can adjust the magnitude signal by the adjustment factor to account for changes in ambient temperature around the electronics of the sensing apparatus.